Abstract

The gold deposit at Porgera is located in the highlands of Papua New Guinea. Alluvial gold was first recognized in 1939, but systematic exploration did not begin until 1961. The middle Miocene Porgera intrusive system, which was derived from melting of thickened crust, was emplaced within Late Cretaceous sediments on the northern margin of the Papuan platform, about 25 km south of the Central orogenic belt. Mineralization is associated with several medium- to fine-grained, high-level, porphyritic intrusions of mafic diorite composition with alteration assemblages of calcite, dolomite, chlorite, and sericite plus subordinate clays and quartz. Sediments on the margin of the intrusions are also altered. Sulfide mineralization consisting of pyrite, sphalerite, and galena is hosted by both igneous and sedimentary rocks and occurs as veins, disseminations, and breccia matrix. The distribution of sulfides, hydrothermal alteration, and associated gold and silver mineralization are largely a function of the intensity and extent of fracturing which is best developed at intrusive contacts and in breccias. Most of the gold mineralization is hosted by sulfides, in particular pyrite and arsenical pyrite. About 60 percent of the gold is apparently submicroscopic and not readily recoverable. On the basis of the Au, Ag, Cu, Zn, and Pb contents of drill core samples five mineralization types have been defined. They reflect several episodes of gold mineralization from older, highly refractory, to younger, less refractory types. Although overprinting is evident, discrete zones dominated by one mineralization type occur and commonly transgress rock types and alteration assemblages. Subdivision of the deposit on the basis of mineralization type and the use of geostatistical procedures has resulted in improved precision of reserve estimates.